It is known to provide electrical contact devices such as slip ring arrangements for use particularly in space on satellites. For example, such a slip ring arrangement may involve a ring which contains a groove which acts as a reservoir for a metal which is liquid during operation and retained in the groove by surface tension. A probe which acts as a brush for the slip ring arrangement is immersed in the liquid metal and thereby makes electrical contact between the probe and the ring. One of the problems in preparing liquid metal electrical contact devices is to apply the charge of liquid metal in such a way that it wets the electrodes thereby making good electrical contact with it.
Prior methods of wetting are based generally upon means whether mechanical or chemical to remove the non-wetting oxide of the metals of which the slip ring and probe or brush are made, from the portions to be wet by the liquid metal. Failure to do so may sometimes cause the liquid metal falsely to wet an oxide or hydrated oxide layer of the liquid metal on such portions over the parent metal of the electrode. Such false wetting tends to dewet in a vacuum environment and result in undesirable operation such as poor or erratic contacts. For example, a mechanical means of oxide removal and liquid metal wetting consists of machining the final configuration while the surfaces are submerged in gallium or in an inert atmosphere. This may be done with a cutting tool, or using a hand-held material with some abrasive properties. It is difficult if not impossible to avoid undesirable impregnation of the electrode with impurities when using such a process.
Chemical means of oxide removal may consist of acid removal of the parent metal oxide or heating those parts of the liquid metal contacting surfaces in vacuum or in an inert atmosphere to a temperature which will drive off the oxide. Then the surfaces must be kept from exposure to any oxygen-containing atmosphere prior to being wet by the liquid metal.
Both the mechanical and chemical methods described above preclude wetting the electrodes or slip ring parts after assembly because the lead-in wires and insulation materials are generally destroyed in the process.
When using the mechanical methods it is difficult to keep the gallium or liquid metal in contact with the cutting tool and the slip ring or brush and also results in troublesome chips and requires relatively large amounts of liquid metal. These mechanical methods do not lend themselves readily for use with any except simple shapes. The mechanical methods are therefore relatively difficult in procedures and require considerable preparatory effort and also are costly in terms of a required amount of liquid metal.
The chemical means of oxide removal also involve assuring that the parent metal be in contact with the liquid metal prior to exposure to oxygen-containing atmosphere. This requirements demands a facility or apparatus such as a dry box or vacuum chamber and further apparatus to apply the liquid metal. In addition, precautions must be taken to be sure that the parent metal is wet in the desired portions only, otherwise additional effort is needed to remove excess liquid metal from contact with undesired areas.